Electrical coil and method of manufacturing



Jam 10, 1967 H. 1'. JONES ELECTRICAL COIL AND METHOD OF MANUFACTURINGFiled April 7, 1965 [vi mm: ffm/z/ f Jaw:

United States Patent 3,297,970 ELECTRICAL COIL AND METHOD OFMANUFACTURING Howard T. Jones, Flat Rock, N.C., assignor to GeneralElectric Company, a corporation of New York Filed Apr. 7, 1965, Ser. No.446,269 4 Claims. (Cl. 336-205) The present invention relates toelectrical coils, and more particularly concerns insulated electricalcoils subject to relatively high voltage conditions and to a method ofmaking such coils.

The invention is applicable, for example, to electromagnetic coilsemployed in series-connected lamp ballast transformers wherein theprimary coil may be subjected to as much as 10,500 volts. In general,the invention will be found useful for insulating electrical deviceswhich must withstand operating voltages in the range of about 600 toabout 20,000 volts.

It is an object of the invention to provide electrical devices such aselectrical coils, and panticularly electromagnetic coils for use inballast transformers, having improved insulation to withstand highvoltage conditions.

It is another object of the invention to provide a method of makingelectrical devices, and especially electrical coils of the above type,and particularly a method of insulating such coils which is relativelysimple and inexpensive, which lends itself for use in large scaleproduction procedures, and provides insulated coils having excellentelectrical properties and relatively smaller size than similar coilsmade by previously employed insulating methods.

Other objects and advantages will become apparent from the followingdescription and the appended claims.

With the above objects in view, the present invention in one of itsaspects comprises an insulated electrical device comprising, incombination, a wound coil of an elongated conductor having an insulatingresin coating thereon, the coil being coated with an insulating varnishwhich permeates the interstices of the coil winding, a first insulatingsheet material comprising a polyester resin cured in situ by heat andpressure wrapped around the coil winding, and a second insulating sheetmaterial comprising a shrinkable polymer material wrapped around thecoil and overlying the first insulating sheet material, the secondinsulating sheet material being heat-shrunk in situ and therebycompressing the first insulating sheet material to provide a fluidtight, dense insulating covering for the coil.

The invention will be better understood from the follow ing description,taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is view of a transformer primary coil winding to which theinvention'is applicable;

FIGUREZ is a view of the FIG. 1 coil provided with an initial insulatingwrapping in accordance with the invention;

FIGURE 3 is a view of the FIG. 2 coil showing a second insulatingwrapping overlying the initial wrapping; and

FIGURE 4 is a view of an assembled transformer unit including theprimary coil insulated in accordance with the present invention.

Referring now to the drawing, and particularly to FIG. 1, there is shownan electrical coil 1 adapted for use as the primary winding of a seriesballast transformer and formed of a helically wound coil of wire andhaving projecting leads 2 and 3 at opposite ends of the winding coveredwith suitable insulating sleeves. The wire is typically composed of acopper strand coated with a suitable insulating material such aspolyvinyl formal or, preferably, the polyester material disclosed inPrecopio et al. Patent 2,936,296 and sold under the trademark Alkanex.The coil is precision wound in a plurality of layers by known methodsand winding apparatus, and is formed with a central opening therethroughfor receiving the secondary coil winding and for inserting this assemblyon the transformer core as hereinafter described.

In accordance with the process of the invention, coil 1 having its turnsbound in place by spaced adhesive strips 4, such as polyester tape orother suitable taping material, tightly encircling the winding layersand adhesively attached thereto, is dipped in an insulating varnish bathfor impregnating the coil with the varnish. A particular varnishmaterial which has been found satisfactory is the thermosettingpolyester resin which is disclosed in the Precopio et a1. patentmentioned above and which comprises the product of reaction of (1) alower dialkyl ester of terephthalic or isophthalic acid, e.g., dimethylterephthalate, (2) ethylene glycol, and (3) a saturated aliphaticpolyhydric alcohol having at least three hydroxyl groups, e.g.,glycerin. For brevity, this polyester resin will herein be referred toas Alkanex.

Other known or suitable insulating varnishes, however, may alternativelybe used, such as oleo-resinous varnishes, oil-modified phenolics,styrene-modified polyester varnishes, and alkyd resins. The Alkanexvarnish described is preferred, however, for its thermal stabilitycoupled with excellent mechanical, chemical and electrical properties.

In a usual procedure, coil 1 is preheated to above 190 C. before thevarnish treatment, and it is then dipped into an Alkanex bath at roomtemperature for 5 minutes, or for a sufiicient period to enable theliquid varnish to permeate into the interstices of the coil turns. Thecoil is then removed from the bath, and the excess varnish allowed todrain oflF. The thus treated coil is then heated in an oven at about C.for about 3 hours to cure the varnish, after which the coil is forcecooled to room temperature.

After being cooled, the coil is preferably provided with a layer ofpressure-sensitive adhesive coated polyester tape (e.g., Mylar) appliedto opposite edge faces of the coil, so as to insure adequate insulation,especially at the corners, to withstand the high electrical stress towhich the coil is subjected during subsequent electrical testing.

The coil is then wrapped with a sheet of insulating material composedpreferably of glass cloth impregnated with a B stage (i.e., semi-cured)polyester resin, the coil being shown in FIG. 2 wrapped with suchpolyester resin impregnated sheet 5. As shown, the polyester resin sheetis folded into the central opening of the coil so as to conform asclosely as possible to the coil form, and is there held in place byspaced adhesive tapes 6.

The polyester resin which impregnates sheet 5 is preferably a completelypolymerizable resin composed of a copolymer of an unsaturated alkydresin and a vinyl mono mer. Such copolymers and the sheet materialincorporating the same which may be employed in practicing the presentinvention are disclosed in the patent to Loritsch 2,528,235, and thedisclosure of the latter patent is accordingly incorporated herein byreference.

As there disclosed, the polyster resin composition may be formed from amixture which includes (1) a polymerizable unsaturated alkyd resinobtained by the esterification reaction of a mixture of ingredientscomprising a polyhydric alcohol and a polycarboxylic acid, (2) apolyallyl ester, and (3) a catalyst for accelerating thecopolymerization of the ingredients of 1) and (2). Unsaturated alkydresins which may be used are, for example, diethylene glycol maleate,diethylene glycol maleate phthalate, glyceryl itaconate, glycerylmaleate, ethylene glycol maleate, diethylene glycol itaconate, propyleneglycol fumarate, triethylene glycol maleate, esterification products ofethylene glycol, itaconic and acid and phthalic anhydride, of diethyleneglycol, itaconic acid and suc cinic acid, of ethylene glycol, maleicanhydride and adipic acid, of diethylene glycol, maleic anhydride anditaconic acid, of diethylene glycol, maleic anhydride and stearic acid,of glycerine, maleic anhydride and phthalic anhydride, of glycerine,maleic anhydride and octyl alcohol, etc. Other examples of polymerizableunsaturated alkyd resins that may be used are given, for example, onpage 4 of DAlelio Patent No. 2,308,495, issued January 19, 1943.

As the polyallyl ester ingredient there may be used, for example,diallyl phthalate, the diallyl esters of carbonic, oxalic, malonic,succinic, glutaric, adipic, pimelic, suberic, sebacic, azelaic, maleic,fumaric, itaconic, benzoyl phthalic, mesaconic, citraconic, tartronic,malic, gluconic, tartaric, isophthalic, terephthalic,benzophe'none-2,4'-dicarboxylic, etc., acids; diand tri-allyl citrates,tricarballylates, aconitates, phosphates, silicates, etc.; tetra-allylsilicate, the tetra-allyl ester of benzene tetracarboxylic acid, etc. v.

The foregoing resin mixture preferably includes a catalyst such asbenzoyl peroxide, or other organic and inorganic peroxides as disclosedin the Loritsch patent, as well as the other catalysts disclosed byLoritsch.

While the inclusion of a polyvinyl acetal resin such as polyvinyl formalmay provide optimum results, as taught by Loritsch, such a component isnot absolutely necessaryin the polyester resin wrapping sheet employedin accordance with the invention.

In a particular B stage polyester resin sheet material found suitablefor purposes of the invention, the sheet material is composed of glasscloth impregnated with a polyester resin comprising a mixture of equalparts of diallyl phthalate and diethylene glycol maleate and a smallamount of a polymerization catalyst, e.g., benzoyl peroxide.

The polyester resin impregnated sheet is such that prior 4 of shrinkingwhen heated. The method of making such shrinkable polymer tape isdisclosed, for example, in the patent to Mathes .et al. 2,929,744, andthe disclosure of the latter patent is accordingly incorporated byreference herein. In accordance with the invention, the coil wrappedwith irradiated polyethylene tape 7 is heated to a temperature of about190 C. which results in shrinking the tape, thus applying pressure tothe underlying poly- I ester resin wrapping sheet so that the latter,under the heat and pressure applied, cures to a hard, strong, dense,

fluid-tight, highly insulating covering for the wound coil.

Such heating of the irradiated polyethylene tape to shrink the sameisadvantageously carried out, in accordance with another aspect of theinvention, after the fully to use, the resin preferably is in dry,uncured, or semicured, slightly tacky condition and the sheet isflexible and conformable to the surface to which it is applied, onlyheat and pressure being required to cure the resin and harden the sheetinto a tought, dense bonded structure. While glass fiber cloth ispreferred for the base material of the sheet, other base sheet materialmay be employed, such as asbestos material, nylon cloth, or otherequivalent material suitable for use as a wrapping sheet. It will beunderstood that if desired the wrapping sheet may be in the form of tapeand wound around the coil as described in connection with wrapping tape7 below.

While the above described polyester resin sheet material is particularlysuitable for purposes of the invention, wrapping sheet having otherinsulating resin material incorporated therein, such as epoxy resins ofknown type, may be used, such resins being condensation products ofpolyhydroxy compounds (such as polyhydric phenols and polyhydricalcohols) and epichlorhydrin. Such epoxy resins impregnated in the sheetmay be liquid epoxides in the B stage, or uncured solid epoxidesdeposited in -sol vent solution on the base tape.

Following the wrapping of the coil with the polyester sheet (orequivalent sheet) the coil 1 may bewrapped around its outside surface,if necessary or desirable to provide added insulation, with a sheet ofmicarnat, i.e., a sheet of reconstituted mica paper, but this step isoptional so far as the present invention is concerned.

The coil is then tightly wrapped, as shown in FIG. 3,

wrapped primary coil is assembled with a secondary coil and atransformer core. In such procedure, the secondary or low voltage coil,which typically is somewhat longer than the primary coil to provideextra electrical creep distance in the transformer assembly and which isnot provided with a wapping as on the primary coil, is nested within thewrapped primary coil with a sheet of micamat arranged between the twocoils. The two coils are then assembled on a transformer core. As shownin FIG. 4, such assembly comprises the wrapped primary coil 1 and nestedsecondary coil 9 with a sheet of micamat 10 arranged therebetweeninserted over a leg of transformer core 11 with the yoke 11a closing theopen end of the transformer core. 7

This transformer assembly is then preheated to abou 190 C. and dipped inan insulating varnish bath such as Alkanex, as previously described inconnection with the varnish impregnation of the unwrapped primary coil.After removal of the assembly from the varnish bath, the excess varnishis allowed to drain, and the assembly is cured in an oven for 3 hours atabout 125 C. to cure the varnish, after which the assembly is cooled.

Such treatment provides a number of desirable results. It cures tohardness the B stage polyester resin sheet material constituting theunderwrap on the primary coil; it shrinks the irradiated polyethylenetape, providing pressure on the underlying polyester sheet wrapping forimproved cure of the latter and promoting its optimum insulating Yproperties, it impregnates the secondary coil with an inwith a strip ofirradiated polyethylene tape 7 to provide is then Wound around the outercircumference of the coil to hold the wrapping turns in place. Theirradiated polyethylene tape 7 hasthe characteristic sulating varnishand thereby eliminates the air voids therein; it impregnates thetransformer core with the insulating Varnish; and it bonds allcomponents of the transformer assembly together in a unitary,mechanically stable, thermally and chemically resistant, well-insulatedstructure.

While particularly satisfactory results have been obtained with the useof irradiated polyethylene tape as the shrinkable wrapping material,other types of shrinkable polymer tape or wrapping material may be used,such as post-oriented polyethylene terephthalate for the purpose ofapplying a contracting force on the under layer of poly ester resinsheet material, such post-oriented polyethylene terephthalate beingdisclosed in detail in the patent 'to Marshall 2,993,820.

It will be understood that while the present invention has beendescribed mainly with respect to the primary winding of a ballasttransformer, the process of the invention is applicable to various typesand forms of electrical devices which may be advantageously insulated bythe disclosed multiwrap insulating procedure of the present invention toimprove their resistance to high voltage conditions.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications maybe made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An insulated electrical device comprising, in com bination, a Woundcoil of an elongated conductor having an insulating resin coatingthereon, said coil being coated with an insulating varnish whichpermeates the interstices of the coil winding, a first insulating sheetmaterial comprising a porous inorganic sheet impregnated with athermosetting resin cured by heat and pressure and wrapped around thecoil winding, and a second insulating sheet material comprising ashrinkable polymer material wrapped around said coil and overlying saidfirst insulating sheet material, said second insulating sheet materialbeing heat shrunk in situ and thereby compressing said first insulatingsheet material to provide a fluid tight, dense insulating covering forsaid coil, the thus wrapped coil having a coating thereon of heat-curedinsulating varnish.

2. An insulated electromagnetic device comprising, in combination, anelectromagnetic core member, a low voltage secondary coil surroundingsaid core member, and a high voltage primary coil surrounding saidsecondary coil, said primary coil being formed of a Wound elongatedconductor having an insulating resin coating thereon, said primary coilbeing coated With an insulating varnish which permeates the intersticesof the coil Winding, a first insulating sheet material comprising aporous inorganic sheet impregnated with a polyester resin cured by heatand pressure and Wrapped around said primary coil winding, a secondinsulating sheet material comprising a shrinkable polymer materialWrapped around said coil and overlying said first insulating sheetmaterial, said second insulating sheet material being heat-shrunk insitu and thereby compressing said first insulating sheet material toprovide a fluid tight, dense insulating covering for said primary coil,the assembly of said core member and coils having a coating thereon of aheat-cured insulating varnish material.

3. An insulated electromagnetic device comprising, in combination, anelectromagnetic core member, a low voltage secondary coil surroundingsaid core member, and a high voltage primary coil surrounding saidsecondary coil, said primary coil being formed of a wound elongatedconductor having an insulating resin coating thereon, said primary coilbeing coated with an insulating varnish which permeates the intersticesof the coil winding, a first insulating sheet material comprising aporous inorganic sheet impregnated with a polyester resin cured by heatand pressure and wrapped around said primary coil winding, a secondinsulating sheet material comprising a heat shrinkable polymer materialcomposed of irradiated polyethylene Wrapped around said primary coil andoverlying said first insulating sheet material, said second insulatingsheet material being heat-shrunk in situ and thereby compressing saidfirst insulating sheet material to provide a fluid tight, denseinsulating covering for said primary coil, the assembly of said coremember and coils having a coating thereon of a heat-cured insulatingvarnish material.

4. The method of making a transformer which comprises forming a firstwound coil of an elongated conductor having an insulating coatingthereon, applying an insulating varnish to said coil to bond the windingturns together and fill the voids therebetween, Wrapping a firstinsulating sheet material comprising a porous inorganic sheetimpregnated with a semi-cured heat and pressure hardenable polyesterresin around the thus treated coil, wrapping a second insulating sheetmaterial comprising a heat shrinkable polymer material composed ofirradiated polyethylene around said coil overlying said first insulatingsheet material, inserting a second coil Within said wrapped insulatedcoil, assembling said coils on a transformer core member, heating saidassembly of coils and core for shrinking said second insulating sheetmaterial, whereby said first insulating sheet material is cured by theheat and pressure thus applied to provide a fluid-tight, denseinsulating covering for said first coil, and applying an insulatingvarnish to said assembly of core and coils.

References Cited by the Examiner UNITED STATES PATENTS 2,929,744 3/1960Mathes et al ll7138.8 X 2,993,820 7/1961 Marshall 15686 3,048,651 8/1962Howard et al. 174l20 LARAMIE E. ASKIN, Primary Examiner. C. A. TORRES,T. J. KOZMA, Assistant Examiners.

1. AN INSULATED ELECTRICAL DEVICE COMPRISING, IN COMBINATION, A WOUNDCOIL OF AN ELONGATED CONDUCTOR HAVING AN INSULATING RESIN COATINGTHEREON, SAID COIL BEING COATED WITH AN INSULATING VARNISH WHICHPERMEATES THE INTERSTICES OF THE COIL WINDING, A FIRST INSULATING SHEETMATERIAL COMPRISING A POROUS INORGANIC SHEET IMPREGNATED WITH ATHERMOSETTING RESIN CURED BY HEAT AND PRESSURE AND WRAPPED AROUND THECOIL WINDING, AND A SECOND INSULATING SHEET MATERIAL COMPRISING ASHRINKABLE POLYMER MATERIAL WRAPPED AROUND SAID COIL AND OVERLYING SAIDFIRST INSULATING SHEET MATERIAL, SAID SECOND INSULATING SHEET MATERIALBEING HEAT SHRUNK IN SITU AND THEREBY COMPRESSING SAID FIRST INSULATINGSHEET MATERIAL TO PROVIDE A FLUID TIGHT, DENSE INSULATING COVERING FORSAID COIL, THE THUS WRAPPED COIL HAVING A COATING THEREON OF HEAT-CUREDINSULATING VARNISH.